In a dot matrix printer, each character is reproduced as a series of dots arranged in accordance with a predetermined format. One common commercial format, usually referred to as the 5.times.7 format, comprises seven horizontally extending rows of dots in five vertical columns. Usually, three blank columns are left between individual characters. Another common format is a 7.times.9 dot array. With either format, all capital letters, all numerals, most lower case letters, and many forms of punctuation can be readily and effectively reproduced in highly readable form.
There are a variety of other characters, however, that produce substantial technical problems in dot matrix printers. These are the lower case letters p, j, y, q, and g, having descender elements that project below the base line of a line of type. In this same category are some punctuation marks such as commas and semi-colons, subscripts, and, in some instances, portions of fractions. For these characters, referred to herein as "descender characters," the usual vertical array of print dot positions in a standard character format is inadequate because there are no print elements available to print dots below the base line.
It has been common practice, in dot matrix printers, to print the descender characters completely above the horizontal base print line. This results in an appearance that does not conform to ordinary practice and is unsatisfactory in many applications, particularly where appearance and ease of readability are required.
Another known technique for printing descender characters entails the addition of two or more print rods to the printer, located below the vertical array used for printing the standard format. This construction materially increases the weight and inertia of the print head, which are already critical factors in a high speed printer. In addition, the cost of the print head is substantially increased, because the print rods, print rod guides, and print rod actuating magnets, particularly in high speed printers, are precision mechanisms for which the cost is quite appreciable. Examples of dot matrix printers that provide additional print rods for printing descender characters are shown in U.S. Pat. Nos. 3,426,880 and 3,627,096.
Another arrangement for printing descender characters in a dot matrix printer, one that does not use additional print rods for printing below the normal base line, is shown in U.S. Pat. No. 3,759,359. In that device, the print head is pivoted downwardly to print each descender character, the downward movement of the print head being equal to the height of two printed dots. This arrangement is useful only if the printer has a large diameter platen. Otherwise, the displacement of the print head from the normal position for printing above the base line to the position in which descender elements are printed below the base line changes the relative spacing between the tip ends of the print rods and the platen surface, so that the top print rods are located much closer to the platen surface than the bottom print rods. Thus, in a printer having a platen of small diameter, this construction results in a substantial degradation of print quality for the bottom portions of descender characters or in puncturing of the paper for the upper dots in such characters. Further, the cost of the precision print head is again increased.
For printers that operate at relatively low print rates, such as the standard telex rate of six characters per second, a large platen may not be particularly disadvantageous. For higher speeds, however, a large diameter platen introduces rather severe problems. The inherently greater weight required for a large platen imposes a requirement for higher torque on the line feed mechanism of the printer. The greater inertia of a large platen may result in an undesirable lengthening of the time required for the line feed operation. The rapid movement of the platen necessary to advance the paper from a masked position behind a ribbon or other masking portion or the printer to a viewing position, as in the text display control described and claimed in U.S. Pat. No. 3,844,395, makes a large diameter platen quite undesirable. Moreover, a large platen inherently increases the overall size of the printer, which is quite undesirable in many applications.